1. Field of the Invention
The present invention relates to a non-linear distortion detection method which detects a non-linear distortion in the power amplifier of a radio communication transmitter, and a distortion compensation amplifying device which compensates the non-linear distortion, and more particularly to a non-linear distortion detection method and a distortion compensation amplifying device in which, even if the band of a modulation signal is widened, a sampling frequency need not be heightened, and distortion detection can be performed without increasing a circuit size and dissipation power.
2. Description of the Related Art
Consideration for an environment, the decrease of consumption power, the decreases of a size and a weight, etc. have been required of a power amplifier in a transmission apparatus, and in order to meet these requirements, it has become necessary to attain a higher efficiency by decreasing thermal radiation. In general, in order to heighten the efficiency of the power amplifier, the peak power of a modulation signal is designed so as to become the saturation power of the power amplifier. However, the intermodulation distortions of a transmission signal appear on account of the non-linearity of the power amplifier, and they interfere with another radio equipment.
The intermodulation distortions ascribable to the power amplifier will be explained. FIG. 7 is an explanatory diagram showing the output spectrum of the power amplifier having a non-linear characteristic (W-CDMA, two carriers (detuning frequency: 5 MHz)).
As shown in FIG. 7, when a modulation signal is inputted, the spectrum spreads due to a non-linear distortion, and the intermodulation distortions (IM3, IM5) appear. As also seen from the figure, the intermodulation distortions appear at the same frequency intervals as the detuning frequency of the modulation signal. A distortion compensation has been extensively performed in order to improve the intermodulation distortions.
Here, predistortion being one of distortion compensation systems will be explained.
The predistortion is a method wherein the reverse characteristic of the power amplifier is provided at a preceding stage, thereby to decrease the intermodulation distortions, and the reverse characteristic is adaptively controlled in accordance with a temperature change and an individual difference.
A prior-art distortion detection method employed in the adaptive control of the predistortion will be described with reference to FIG. 8. FIG. 8 is a block diagram of a power amplifying device which employs the prior-art distortion detection method.
As shown in FIG. 8, the prior-art power amplifier is configured of a predistorter 1, a D/A converter 2, an orthogonal modulator 3, an oscillator 4, a power amplifier 5, a directivity coupler 6, a mixer 7, an oscillator 8, an A/D converter 9, a distortion detection unit 12 and a control unit 13. The distortion detection unit 12 is further configured of an FFT calculation portion (in the figure, FFT) 10 and an IM calculation portion 11.
The predistorter 1 performs a distortion compensation which bestows the reverse characteristic of the non-linear distortion on an input signal, in compliance with a command from the control unit 13.
The D/A converter 2 converts the distortion-compensated digital input signal into an analog signal.
The oscillator 4 oscillates an RF frequency.
The orthogonal modulator 3 subjects the inputted analog signal to orthogonal modulation, thereby to up-convert the signal at the frequency of the oscillator 4.
The power amplifier 5 amplifies the inputted RF signal at a predetermined amplification factor, and outputs the amplified signal.
The directivity coupler 6 branches the output signal from the power amplifier 5, so as to feedback the branched signal.
The mixer 7 synthesizes a signal from the oscillator 8 and the signal branched from the directivity coupler 6, thereby to down-convert the synthesized signal into an IF frequency.
The A/D converter 9 A/D-converts the down-converted signal with a clock-2 (CLK2), so as to sample the resulting signal.
The distortion detection unit 12 detects a distortion contained in the inputted sampling signal, and outputs the detected distortion as a distortion value to the control unit 13.
The FFT calculation portion 10 of the distortion detection unit 12 obtains a spectrum by subjecting the inputted signal to an FFT (Fast Fourier Transform).
The IM calculation portion 11 calculates the frequencies of intermodulation distortions from the number of carriers of a modulation signal and the detuning frequency thereof, and it outputs power values at the frequencies, to the control unit 13 as distortion values on the basis of the spectrum.
In addition, the control unit 13 adaptively controls the predistorter so that the inputted distortion values may become smaller.
Operations in the power amplifier of the above configuration will be described.
The input signal of the IF frequency inputted in a digital I/Q form is endowed with the reverse characteristic of the non-linear distortion of the power amplifier by the predistorter 1, it is converted into the analog signal by the D/A converter 2, and the analog signal is subjected to the orthogonal modulation and is up-converted into the RF frequency by the orthogonal modulator 3, and the resulting signal is amplified at the predetermined amplification factor and is outputted by the power amplifier 5.
On the other hand, part of the output of the power amplifier 5 is derived by the directivity coupler 6 and is down-converted into the IF frequency by the mixer 7, the IF frequency is converted into the digital signal by the A/D converter 9, the digital signal is subjected to the spectrum detection by the FFT calculation portion 10 of the distortion detection unit 12, and the power values at the intermodulation distortions (IM3, IM5) are calculated by the IM calculation portion 11 and are outputted to the control unit 13 as the distortion values.
In addition, the control unit 13 adaptively controls the predistorter so as to make the distortion values smaller.
It is at odd-ordered distortions that the non-linear characteristic of the power amplifier appears as the intermodulation distortions. Therefore, processing in the predistorter which bestows the non-linear reverse characteristic of the power amplifier can be approximated by Formula (1):y=x+α·|x|2·x+β|x|4·x+γ·|x|6·x  Formula (1)
Here, x and y denote the input signal and output signal of the predistorter, respectively, and the signals are complex numbers. The control unit 13 controls the values of α, β and γ by the use of a perturbation method so that the distortion values obtained by the distortion detection unit 12 may become smaller.
Here, the schematic configuration of the predistorter 1 will be described with reference to FIG. 9. FIG. 9 is a block diagram showing the schematic configuration of the predistorter 1.
As shown in FIG. 9, the predistorter 1 includes a plurality of multipliers and an adder, and it is configured so as to calculate the components of the third power, fifth power and seventh power from the input signal (x), to multiply the respective components by the coefficients α, β and γ and to obtain the output signal (y) on the basis of Formula (1).
α, β and γ are complex numbers, and they are respectively represented as:α=A3·exp(j*Φ3)β=A5·exp(j*Φ5)γ=A7·exp(j*Φ7)  Formulas (2)
In the control unit 13, therefore, these coefficients are cyclically controlled by the perturbation method in the sequence of Φ3→A3→Φ5→A5 Φ7→A7→Φ3.
The control employing the perturbation method, in the control unit 13 will be described with reference to FIG. 10. FIG. 10 is a flow chart diagram showing the control employing the perturbation method, in the control unit 13.
As shown in FIG. 10, when a process is started, the control unit 13 first sets the coefficient to-be-updated (K, and here, the first coefficient is Φ3) and fetches the number of times of setting and the last distortion value as initialization (100).
In addition, when the current distortion value calculated in the distortion compensation unit 12 is inputted, the control unit 13 compares the magnitudes of the current distortion value and the last distortion value (101). If the current distortion value has become smaller (in case of “Yes”), the coefficient is further updated in the same direction (K=K+Step) (103).
Besides, if the distortion value has become larger at the processing 101 (in case of “No”), the control unit 13 inverts an updating direction (Step=Step*(−1)) (102), and it shifts to the processing 103 so as to update the coefficient.
Subsequently, the control unit 13 counts how many times the same coefficient (here, Φ3) was successively updated (104), and it retains the distortion value detected as the “current distortion value” at the processing 101 (105). The distortion value retained here is used as the “last distortion value” at the processing 101 of the next time.
In addition, the control unit 13 compares the stored number of times of updating and the number of times of setting as has been set at the initialization of the processing 100 (106) If the number of times of updating is equal to or less than the number of times of setting, the control unit 13 returns to the processing 101 and repeats the updating of the coefficient Φ3.
Besides, in a case where the number of times of updating exceeds the number of times of setting at the processing 106, the control unit 13 alters the coefficient to-be-updated (107). Here, the coefficient to-be-updated is altered from Φ3 to A3. In addition, the control unit 13 clears the stored number of times of updating (108).
In the control unit 13, the coefficients of the predistorter are controlled so as to make the distortion value smaller, by the control employing such a perturbation method. In this way, the non-linear reverse characteristic in the power amplifier can be approximated by the predistorter employing the power series, and the distortion compensation is permitted.
Incidentally, as a prior-art technique of a transmission apparatus which performs the distortion compensation, there is JP-A-2005-20515 laid open on Jan. 20, 2005, “ADAPTIVE PREDISTORTER TYPE DISTORTION-COMPENSATED TRANSMISSION APPARATUS AND METHOD FOR SWITCHING DELAY CONTROL FILTER COEFFICIENTS THEREFOR” (Applicant: Fujitsu Ltd., Inventor: Mitsuharu Hamano).
This prior-art technique is a method wherein, in switching the filter coefficients of a delay control filter which adjusts the phases of a transmission signal and a feedback signal in an adaptive predistorter type distortion-compensated transmission apparatus, the filter coefficient to be set anew is read out from a memory in which the filter coefficients are stored beforehand, and it is distributed to a filter coefficient setting register via a test loop path which is branched from a main signal loop path for conveying the transmission signal and which is folded back, whereby the switching of the filter coefficients can be performed at a high speed (refer to Patent Document 1).
Besides, as another prior-art technique, there is JP-A-2005-102029 laid open on Apr. 14, 2005, “ADAPTIVE TYPE PREDISTORTER” (Applicant: Mitsubishi Electric Corporation, Inventor: Ken-ichi Horiguchi).
This prior-art technique updates a compensation coefficient in a distortion compensation circuit and attains a stable convergence characteristic irrespective of the amplitude level of an input signal, etc., in conformity with a normalized least square mean algorithm wherein a comparator detects the error between an output signal from a distortion compensation circuit and an input signal to the distortion compensation circuit, and wherein a normalized least square mean circuit minimizes the square mean of error signals by normalizing the square mean with the variance of the input signal (refer to Patent Document 2).
Further, as another prior-art technique, there is JP-A-2005-73032 laid open on Mar. 17, 2005, “DISTORTION COMPENSATION AMPLIFYING DEVICE AND DISTORTION COMPENSATION METHOD” (Applicant: Hitachi Kokusai Electric Inc., Inventor: Naoki Hongo).
This prior-art technique is such that a control unit performs the curve interpolations of a plurality of points which are stored by distortion compensation table means for storing predistortion magnitudes corresponding to electric power values, in a plurality of intervals which overlap partly, and that the control unit joins individual curves obtained by the curve interpolations, thereby to update the points which are stored by the distortion compensation table means, and to compensate a distortion characteristic which contains an inflection point (refer to Patent Document 3).
Besides, as a technique which controls a predistorter on the basis of the equalization error of an equalizer, there is US20050163249A1 (refer to Patent Document 4). Besides, as a technique which includes an equalizer for extracting a distortion, and an equalizer for compensating the distortion, there is “Lei Ding et al, Memory Polynomial Predistorter Based on the Indirect Learning Architecture, GLOBECOM 2002-IEEE Global Telecommunications Conference, no. 1, Nov. 2002 pp. 976-980” (refer to Non-patent Document 1). In addition, as techniques concerning the distortion compensation, there are US20050163250A1, US20050099230A1 and US20050089125A1 (refer to Patent Documents 5, 6 and 7).    Patent Document 1: JP-A-2005-20515 (pp. 4-8)    Patent Document 2: JP-A-2005-102029    Patent Document 3: JP-A-2005-73032    Patent Document 4: US20050163249A1    Non-patent Document 1: Lei Ding et al, Memory Polynomial Predistorter Based on the Indirect Learning Architecture, GLOBECOM 2002-IEEE Global Telecommunications Conference, no. 1, Nov. 2002 pp. 976-980    Patent Document 5: US20050163250A1    Patent Document 6: US20050099230A1    Patent Document 7: US20050089125A1
With the prior-art power amplifying device, however, the signal of the output of the power amplification portion is subjected to the frequency conversion by the FFT, and the electric powers of the intermodulation distortions are calculatively obtained, thereby to detect the distortion. It is therefore necessary to perform the sampling and the signal processing as to a frequency range which covers the bands of the intermodulation distortions.
A case where signal processing of wide band is required, will be explained with reference to FIG. 11. FIG. 11 is an explanatory diagram showing another output spectrum of a power amplifier having a non-linear characteristic (W-CDMA, two carriers (detuning frequency: 15 MHz)).
As shown in FIG. 11, the intermodulation distortions (IM3, IM5) appear at the same frequency intervals as the detuning frequency of the two carriers. Therefore, when the detuning frequency becomes large, signals of wider bands must be processed in the distortion detection unit in order to calculate the electric power values by detecting the spectra of the distortions IM3 and IM5.
In the future, a request for high-speed transfer will inevitably rise, and it is expected that the frequency band of a modulation signal will widen more and more.
Further, as the band of the signal widens more, the sampling frequency needs to be raised in the A/D converter (A/D converter 9 in FIG. 8) for detecting the distortion, and a calculation amount in the FFT calculation portion of the distortion compensation unit enlarges, to incur such problems as the increase of a circuit size, the rise of a cost and the increase of consumption power.
Besides, in a case where, as in Patent Document 1, a transmission signal and a feedback signal are compared in a temporal region so as to detect an error, it has been difficult to bring a phase, an amplitude and a delay time exactly into agreement.